The microsecond temporal precision shown by auditory systems is essential for location and decoding of sound. This grant examines the neural circuits that encode and process temporal information in the barn owl and other birds and reptiles in order to identify shared general features of auditory coding. Temporal information is processed in the cochlear nucleus magnocellularis, which projects to the nucleus laminaris, where interaural time differences (ITDs) are first computed. Since precise temporal coding in the inputs to the nucleus laminaris is critical to the detection of ITDs, we will determine how phase-locking changes during circuit development. These studies will be carried out in parallel with studies of the regulation of myelination, which is also essential for temporal coding. The nature of the neural code for ITD is controversial, with current models advocating either a map-like place code for ITD, consistent with data in the barn owl, or a rate-based population code, consistent with data from small mammals. We will test these theories in the chicken, which has a similar head size and phase locking as small mammals, to address the neural coding strategies for ITD. What is needed for good ITD detection? ITD coding neurons generally possess bipolar morphology in both birds and mammals, with inputs from each ear segregated onto dendritic trees. Since this segregation improves ITD coding in modeling studies, we will use in vitro analyses to look for the predicted dendritic non-linearities in coincidence detector neurons. The barn owl's sound localization is more accurate than a chicken's. We will use in vivo recordings to determine if this is due to computational power (i.e. more neurons devoted to a particular ITD computation) and/or to specific improvements in the responses of individual neurons. The similarity in physiological responses in the cochlear nucleus angularis and mammalian cochlear nucleus suggests there is convergent evolution of cells specialized for encoding relevant features of the auditory stimulus. Comparisons of coding in bird and mammal CN should allow us to identify salient features of the neural codes in the ascending auditory stream, and show how response types emerge.